Twelve centimetres of snow have stopped the traffic around the Maesmynis Valley, near Builth Wells in mid-Wales, and obscured the deep green patchwork of fields. (Nancy Durham/CBC)

Heavy snowfall pounding Britain on Wednesday forced the closure of thousands of schools, disrupted travel and prompted officials to mobilize the army to help stranded motorists.

Britain's weather office has put the entire country on alert, warning of icy roads and very heavy snowfall throughout the U.K.

Delays and cancellations plagued Heathrow Airport, although it remained open. But London Gatwick was closed Wednesday morning for snow clearance, along with Manchester Airport and Liverpool John Lennon Airport.

The ice and snow have also caused delays and cancellations for the country's rail service, with lines in southern England reporting reduced services.

A postman makes his way through heavy snow despite roads blocked in the Huddersfield area of Britain, as snow continues to fall across the country. (John Giles/PA Wite/Associated Press)

Britain's military and coast guard services were brought in to help rescue workers reach hundreds of stranded motorists. Up to 1,000 vehicles were caught in a massive snow-related traffic jam in Hampshire, in southern England.

Many people were evacuated to nearby rescue centres but some people slept in their vehicles overnight.

Only minor disruptions in London

The Automobile Association said it had dealt with 15,000 incidents across the country and sections of the country's major highways were closed.

London was spared the worst of the weather, with only minor disruptions reported in the capital. Most of the capital's bus and subway lines were running.

The snow hit the northern parts of the country, then moved south, with up to 40 centimetres believed to have been dumped by Tuesday.

The national weather office said Britain was experiencing its longest cold snap since 1981.

Temperatures were just below freezing in most areas, falling as low as –10 C in northern Scotland. The unusually cold weather is expected to continue for the next two weeks.

Games postponed

The heavy snow forced soccer team Arsenal to postpone its Premier League game against Bolton in London on Wednesday, one of several sporting events called off because of severe winter weather.

The heaviest snowfalls to hit Britain in three decades had already forced two League Cup games — Tuesday's Blackburn-Aston Villa match and Wednesday's Manchester City-Manchester United game

Both will be rescheduled for later this month.

Celtic's Scottish Cup game on Saturday was also postponed, as were several rugby union games and horse-racing meets.

With files from The Associated Press